I understand that boiling point is a colligative property for aqueous solutions, and that it empirically depends on the mole fraction of the solute, rather than the identity of the solute. I understand the van't Hoff factor / ionic strength and why it matters. I also get that an increase in entropy is the primary driving force for boiling point elevation.

My question is, why does the influence of intermolecular attraction go out the window for solutions, as opposed to pure substances? Shouldn't the identity of the specific dissolved species - that is, their charge and their size, affect the intermolecular attractions in the solution? Shouldn't greater intermolecular attraction increase the boiling point by reducing vapor pressure, just as it does for pure substances? For example, neutral molecular solutes as opposed to strong electrolytes, or large ions like iodide vs smaller ones like fluoride.

Is the effect simply too small to matter compared to the entropy factor, or am I missing something theoretically?

$\begingroup$I think your suspicion that the non-ideal intermolecular attraction influences the colligative properties is justified. Have a look here for example. Especially for ionic solutes there is a difference between the expected (ideal) and the observed behavior. But usually the entopy factor is dominant.$\endgroup$
– PhilippMay 7 '14 at 18:57

3 Answers
3

GOOD QUESTION! That is the nastiest p-chem question I've seen. Atkins, Physical Chemistry 2nd edition (the readable one), pp 228-232pp. Boiling point elevation is presented as purely an entropy effect, given two assumptions:

1) The solute is non-volatile.
2) The non-volatile solute does not dissolve in the solid solvent.
Both assumptions are explored in Chapter 10.

A solute increases the entropy of the solution but does not change the entropy of the vapor at a given P and T. A higher T is then necessary to give the same entropy difference. I am not satisfied that this wholly addresses a strongly interactive solute versus a non-interactive solute versus a strongly negatively interactive solute,

Perhaps the enthalpy of interaction between the solute and solvent does not change as the solvent evaporates (at least until the concentration of solute gets high enough that solute-solute interactions become important) so the entropy is the only factor that changes.

You are not wrong to assume that the solute-solvent interactions matters. They indeed do and will influence on vapor pressure of the liquid. The caveat here is that this property, like many others of solutions, is primarily described for ideal solutions.

In an ideal solution, the solvent-solute, solvent-solvent and solute-solute interactions are all considered identical (Chemical Principles, P. Atkins & L. Jones). So even though the interactions can't be ignored (like it's done for ideal gases), they shouldn't influence on the properties since they are all the same.

It's no surprise then that real solutions will have a closer-to-ideal behavior as they get more and more diluted, since, in practice, there are proportionally fewer and fewer solute-solvent interactions happening.